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United States Patent |
6,018,399
|
Yamada
,   et al.
|
January 25, 2000
|
Image processing device which discriminates effectively between
character pixels and halftone pixels
Abstract
The present invention is directed to an image processing device that can
prevent the occurrence of moire patterns when processing a dot image. An
image processing device according to the present invention comprises an
image memory for storing multi-gradational image data, an area-deciding
portion for deciding an area of pixels PIX (x, y) of a multi-gradational
image stored in the memory and outputting a decision signal SEL=0 when the
pixel is a character area pixel or a decision signal SEL=1 when the pixel
is a halftone area, a binarizing portion for character for binarizing a
character image pixel PIX (x, y) of the multi-gradational image data
stored in the memory and outputting a binarized image data L, a binarizing
portion for photograph for binarizing a halftone image pixel PIX (x, y) of
the multi-gradational image data stored in the memory and outputting a
binarized image data P, an output selecting portion for selectably
outputting binarized image data B which is the binarized image output L
from the binarizing portion for character when the output SEL of the area
deciding portion is 1 or the binarized image output P from the binarizing
portion for photograph when the output SEL is 0, and a memory for storing
binarized image data outputted from the output selecting portion.
Inventors:
|
Yamada; Hideaki (Ichihara, JP);
Takeda; Yuuki (Chiba, JP)
|
Assignee:
|
Sharp Kabushiki Kaisha (Osaka, JP)
|
Appl. No.:
|
931700 |
Filed:
|
September 16, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
358/1.9; 358/462 |
Intern'l Class: |
H04N 001/40 |
Field of Search: |
395/109
382/270-273,224-228,173,176
358/462,466,1.9
|
References Cited
U.S. Patent Documents
5016118 | May., 1991 | Nannichi | 358/462.
|
5018024 | May., 1991 | Tanioka | 358/462.
|
5260810 | Nov., 1993 | Kanno et al. | 358/462.
|
5331442 | Jul., 1994 | Sorimachi | 358/462.
|
Foreign Patent Documents |
59-218076 | Dec., 1984 | JP.
| |
Primary Examiner: Lee; Thomas D.
Assistant Examiner: Brinich; Stephen
Attorney, Agent or Firm: Dike, Bronstein, Roberts & Cushman, LLP, Conlin; David G., Daley, Jr.; William J.
Claims
We claim:
1. An image processing device comprising area deciding means for
discriminating an objective pixel to be of a character image area or a
halftone image area by sequentially checking pixels of multi-gradational
image data, pixel by pixel, character-binarizing means for binarizing
character image areas, photograph-binarizing means for binarizing halftone
image areas, and output means for selectably outputting binarized data for
character and binarized data for halftone according to the area decision
results;
wherein the area deciding means for discriminating the objective pixel to
be of a character image area or a halftone image area by sequentially
checking pixels of multi-gradational image data pixel by pixel decides
which area a particular pixel is in by reference to a set of pixels
consisting of four pixels at the following locations: (a) above and to the
left of a pixel located above and to the left of the particular pixel, (b)
above and to the right of a pixel located above and to the right of the
particular pixel, (c) below and to the left of a pixel located above and
to the left of the particular pixel, and (d) below and to the right of a
pixel located above and to the right of the particular pixel.
2. An image processing device as defined in claim 1, wherein a first memory
for storing the multi-gradational image data is provided.
3. An image processing device as defined in claim 1 or 2, wherein a second
memory for storing the binarized data is provided.
Description
BACKGROUND OF THE INVENTION
In a usual facsimile transmission, a sending terminal reads an original
image as multi-valued image data by an incorporated scanner, converts the
data into binary image data and sends the converted binary data to the
receiving terminal, which outputs the received binary image data for
printing the data on recording paper.
The conventional image processing device converts multi-valued image data
into binary image data in the following way:
First, an original image is divided into character-image areas and halftone
(e.g., photographic) image areas.
Means of discriminating between the character image areas and the halftone
image areas are usually called as area deciding means.
The area decision is made firstly by examining whether an optical density
value of an objective pixel to be examined is within a range between a
specified black threshold and a specified white threshold and secondly by
examining whether a difference of the density value of the objective pixel
from an average density value of four pixels existing in above left, above
right, below left and below right of the objective pixel is equal or
smaller than a specified threshold value. The objective pixel is decided
as a halftone area if said pixel satisfies both conditions as the result
of the examinations. The objective pixel is decided as a character area if
it does not satisfy both conditions at a time as the result of the
examinations.
The halftone pixel thus decided is binarized by a dithering with error
diffusion method. A pixel judged as a character area pixel is binarized by
using a specified threshold value, and omitting the dithering process,
that may impair the quality of character information halftone image
represented by halftone dots of a larger size than a certain value of
roughness cannot be processed without causing moire fringes by a
conventional image processing device. Moire fringes may be produced
because the conventional image processing device periodically mistakes an
image represented by halftone dots for a character image.
In the conventional image processing device, an area of each of the pixels
read by scanning the original image is decided by reference to four pixels
existing in just upper, lower, left and right of each objective pixel. In
this case, each objective pixel is judged to be of a character area if a
difference cyclically exceeds a certain threshold value.
In this halftone dot image, pixels at positions of a certain cycle are
misjudged to be of a character area because the differential values of its
density from an average density value of the reference pixels exceed the
threshold value. These misjudgments cause moire patterns in the image when
reproduced.
SUMMARY OF THE INVENTION
The present invention relates to an image processing device and more
particularly to an image processing device used for converting multivalued
image data into binary image data in a binary-image-data transmission
system such as a facsimile transmission system.
The present invention is directed to an image processing device that
properly decides pixels to be of a character image area and is protected
against the occurrence of moire fringe patterns.
To achieve the above-mentioned object, the present invention provides an
image processing device which comprises area deciding means,
character-binarizing means for binarizing character image areas,
photograph-binarizing means for binarizing halftone image areas (e.g.,
photographic image areas) and output means for selectably outputting
binarized data for character and binarized data for photograph according
to the area decision results, wherein the area deciding means for
discriminating between character areas and halftone areas (e.g.,
photographic image areas) by sequentially checking pixels of
multi-gradational image data pixel by pixel decides an objective pixel
area by referring to four pixels existing in above left of above left,
above right of above right, below left of below left and below right of
below right respectively of the objective pixel.
This image processing device is further provided with a first memory for
storing the multi-gradational image data and a second memory for storing
the binarized data.
In the above-mentioned image processing device, a multi-gradational image
data is stored in the first memory and then subsequently read pixel by
pixel and decided as character area pixels or halftone (e.g., photographic
image) area pixels. Simultaneously, character pixels and halftone (e.g.,
photographic image) pixels are binarized respectively. The binarized data
for character and the binarized data for photograph are selectably
outputted pixel by pixel according to the above-described area-deciding
results. The binarized image data is stored in the second memory.
In the process of subsequently deciding the character areas and halftone
(e.g., photographic image) areas of the multi-gradational image data, each
objective pixel area is decided by reference to four neighboring pixels
existing in above left of above left, above right of above right, below
left of below left and below right of below right of the objective pixel.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is illustrative of a correlation between a dot image and positions
thereon to be read by scanning by a conventional image processing.
FIG. 2 is a block diagram showing a general construction of an image
processing device according to the present invention.
FIG. 3 is a block diagram showing a detailed construction of an area
deciding portion of an image processing device according to the present
invention.
FIG. 4 is a block diagram showing a detailed construction of an edge
deciding portion of an image processing device according to the present
invention.
FIG. 5 is illustrative of a correlation between a dot image and positions
thereon to be read by scanning by an image processing device according to
the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
Prior to explaining preferred embodiments of the present invention, an
image processing device basic art on which the present invention stand,
will be described below as references for the present invention.
In a usual facsimile transmission, a sending terminal reads an original
image as multi-valued image data by an incorporated scanner, converts the
data into binary image data and sends the converted binary data and the
receiving terminal outputs the received binary image data for printing the
data on recording paper.
The conventional image processing device converts multi-valued image data
into binary image data in the following way:
First, an original image is divided into character-image areas and halftone
(e.g., photographic) image areas.
Means of discriminating between the character image areas and the halftone
image areas are usually called as area deciding means.
The area decision is made firstly by examining whether an optical density
value of an objective pixel to be examined is within a range between a
specified black threshold and a specified white threshold and secondly by
examining whether a difference of the density value of the objective pixel
from an average density value of four pixels existing in above left, above
right, below left and below right of the objective pixel is equal or
smaller than a specified threshold value. The objective pixel is decided
as a halftone area if said pixel satisfies both conditions as the result
of the examinations. The objective pixel is decided as a character area if
it does not satisfy both conditions at a time as the result of the
examinations.
The halftone pixel thus decided is binarized by a dithering with error
diffusion method. A pixel judged as a character area pixel is binarized by
using a specified threshold value, and omitting the dithering process that
may impair the quality of character information.
A halftone image represented by halftone dots of a larger size than a
certain value of roughness cannot be processed without causing moire
fringes by a conventional image processing device. Moire fringes may be
produced because the conventional image processing device periodically
mistakes an image represented by halftone dots for a character image.
FIG. 1 shows a relation between a dot image and positions thereon to be
read by a scanner of a conventional video processing device to explain a
reason for causing a moire pattern. In FIG. 1, white circles show
positions readable by the scanner and black circles (with hatching
therein) show halftone dots composing the halftone dot image.
In the conventional image processing device, an area of each of the pixels
read by scanning the original image shown in FIG. 1 is decided by
reference to four pixels existing in just upper, lower, left and right of
each objective pixel. In this case, each objective pixel is judged to be
of a character area if a difference cyclically exceeds a certain threshold
value.
In this halftone dot image of FIG. 1, pixels at positions of a certain
cycle PIX0, PIX1 and PIX2 are misjudged to be of a character area because
the differential values of its density from an average density value of
the reference pixels exceed the threshold value. These misjudgments cause
moire patterns in the image when reproduced.
A preferred embodiment of the present invention will be described.
FIG. 2 is a block diagram showing a general construction view of an image
processing device according to the present invention.
As shown in FIG. 2, this image processing device comprises: a memory 10 for
storing multi-gradational image data; an area deciding potion 11,
connected to the memory 10, for deciding areas of multi-gradational pixels
PIX (x, y) stored in the memory 10 by outputting a signal of SEL=0 when
PIX (x, y) is of a character-image area or a signal of SEL=1 when a pixel
PIX (x, y) is of a halftone-image area;
a binarizing portion for character 12, connected to the memory 10, for
character-binarizing the multi-gradational image data PIX (x, y) stored in
the memory 10 and outputting binarized image data L;
a binarizing portion for photograph 13, connected to the memory 10, for
halftone-image (e.g., photographic image) binarizing the multi-gradational
image data PIX (x, y) stored in the memory 10 and outputting binarized
image data P;
an output selecting portion (selector) 14, connected to the area deciding
portion 11, the binarizing portion for character 12 and the binarizing
portion for photograph 13, for selectably outputting a binarized image
data L from the binarizing portion for character 12 when the area deciding
portion 11 outputs SEL=0 or a binarized image data B to be changed over
the binarized image data P from the binarizing portion for photograph 13
when the area deciding portion 11 outputs SEL=1; and
a memory for binarized image 15, connected to the output selecting portion
14, for receiving and storing binarized image data B outputted from the
output selecting portion 14.
FIG. 3 is a block diagram for explaining the detailed structure of the area
deciding portion 11 shown in FIG. 2.
This area deciding portion 11 comprises: an edge deciding portion 21 that
reads multi-gradational image data, examines whether an objective pixel at
PIX (0, 0) satisfy a specified condition by reference to 4 pixels existing
at PIX (-2, -2), PIX (2, -2), PIX (-2, 2) and PIX (2, 2) shown in FIG. 4
(hereinafter described), and outputs a decision signal C=1 when the
condition is satisfied or a decision signal C=0 when the condition is not
satisfied; a density deciding portion 22 that examines whether the
objective pixel satisfies a specified condition and outputs a decision
signal C=1 when the condition is satisfied or a decision signal C=0 when
the condition is satisfied; and an AND circuit 23 that presents an output
SEL corresponding to a logical AND for inputs E and C.
The density deciding portion 22 judges whether the following conditional
equation (1) is satisfied or not. In the equation (1), Q (0, 0) designates
the density of an objective pixel PIX (0, 0) to be decided as character
area or halftone area. TB designates a threshold value for discriminating
the black density level and TW designates a threshold value for
discriminating the white density level.
TB.ltoreq.Q (0, 0).ltoreq.TW (1)
FIG. 4 is a view for explaining the edge deciding portion 21 shown in FIG.
3. This edge deciding portion 21 refers to 4 pixels existing at PIX (-2,
-2), PIX (2, -2), PIX (-2, 2) and PIX (2, 2) according to Equation (2) (to
be described below) and outputs a decision signal E=1 when said equation
is satisfied or a decision signal E=0 when the equation is not satisfied.
In this case, the density values of pixels PIX (-2, -2), PIX (2, -2), PIX
(-2, 2) and PIX (2, 2) are expressed by Q (-2, -2), Q (2, -2), Q (-2, 2),
and Q (2, 2) respectively. In Equation (2), Q max is a maximum of the
density values Q (-2, -2), Q (2, -2), Q (-2, 2), Q (2, 2) and Q min is a
minimum of the density values Q (-2, -2), Q (2, -2), Q (-2, 2), Q (2, 2).
TE is a threshold value for judging an objective pixel to be of an edge.
Q max-Q min.ltoreq.TE (2)
FIG. 5 shows a dot image and positions thereon to be readable by a scanner
of an image processing device according to the present invention. The
shown image relates to multi-gradational image data used in an embodiment
of the present invention. The multi-gradational image data has the
resolution of 200 digits/inch. A pixel 401 shows a position of a pixel to
be read by the scanner. A black circle 402 is a dot component of a dot
image of a 65 lines/inch. A line memory 403 stores a multi-gradational
image. The shown embodiment requires 5 line memories.
As shown in FIG. 5, each of the pixels in a multi-gradational image read by
the scanner is discriminated to be of a character image area or a halftone
(photographic) image area by reference to four neighboring pixels existing
in above left of above left, above right of above right, below left of
below left and below right of below right of the pixel. This method well
adapted to cover the scanner's reading period of 200 dpi and a dot image
of 65 lines/inch (of, e.g., newspaper) and can discriminate between
halftone image areas and character image areas even in the dot image at an
improved accuracy in comparison with the conventional method.
According to the present invention, an objective pixel area in a
multi-gradational image read by a scanner is decided by reference to
neighboring pixels existing in above left of above left, above right of
above right, below left of below left and below right of below right of
the objective pixel, so the discrimination between character areas and
halftone areas in dot image can be realized at an increased accuracy as
compared with the conventional method.
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